Method for automatic adjustment of anodes

ABSTRACT

An improved method for automatic adjustment of anodes in an electrolytic cell is described which utilizes the measurement of the rate of change in current density for a given unit of distance when a minor fraction of the anodes are lowered toward the liquid cathode. When the change in rate of current density exceeds a predetermined limit, movement of the minor fraction of anodes is stopped, and another minor fraction of anodes is adjusted.

United States Patent 1191 Ralston, Jr. Y

[ METHOD FOR AUTOMATIC ADJUSTMENT OF ANODES [75] Inventor: Richard W. Ralston, Jr., Cleveland,

Tenn.

Assignee: Olin Corporation, New Haven, Conn.

[22] Filed: Apr. 18, 1974 [21] Appl. No.: 461,822

Related US. Application Data 2 [63] Continuation of Ser. No. 272,240, July 17, 1972,

abandoned.

1451 Mar. 25, 1975 p 9/1972 7 Caleffi 204/225 x 3,817,846 6/1974 Gebaueretal. 204/225 x FOREIGN PATENTS OR APPLlCATlONS 1,212,488 11/1970 UnitedKingdom... 204/225 2,000,928 7/1971 Germany 204/99 Primary ExaminerJohn l-l. Mack Assistant Examiner-D. R. Valentine Attorney, Agent, or Firm-Donald F. Clements; Thomas P. ODay; James-B. l-laglind 57 ABSTRACT An improved method for automatic adjustment of anodes in anelectrolytic cell'is described which utilizes the measurement of the rate of change in current density for a given unit of distance whena minor fraction of the anodes are lowered toward the liquid cathode.

[58] Field of Search 204/225 3 3 6 When the change in rate of current density exceeds a predetermined limit, movement vof the minor fraction [56] References cued 'of anodes is stopped, and another minor fraction of UNITED STATES PATENTS amdes adlusted' 3.654118 4/1972 Selwa 204/250 x 19 3 Drawing Figures ELEC TROLY IC CELLS fl 3 1 f CELL SELECTOR UNIT ANODE SE T satscrore MOTOR rL MANUAL CONTROL i CONTROL UNIT TMTTEMZTZ CONTROL 1 UNIT PATENIEUmzsma 73,43

seamlg g fLfCTROLY/C CELLS CELL SELECTOR uN/T r f 4 ANODE SE7 SELECTOR 7 f v MOTOR A MANUAL CONT/POL CONTROL UNIT h QLTTOMZTTC CONTROL UNIT [JG-l METHOD FOR AUTOMATIC ADJUSTMENT OF ANODES This is a continuation of copending application Ser. No. 272,240, filed July 17, 1972, now abandoned.

This invention relates to-an improved method of determining the optimum position of a minor fraction of adjustable anodes in an electrolytic cell containing said anodes and a liquid cathode. More particularly, this invention relates to an improved method of positioning anodes in mercury cells.

Electrolytic mercury cells have been used commercially in the production of chlorine and caustic by the electrolysis of brine for many years. Usually these cells employ a metal cell container which slopes slightly downward from one end to the other, and which utilizes a cathode comprised of a moving stream of mercury on the bottom of the cell. A stream of brine flows on top of the mercury cathode in the cell container. Graphite anodes and more recently, metal anodes, are adjustably secured to the top of the cell'container and positioned in the brine above the mercury cathode. When a voltage is applied across the cell, current flows from the anode, through the brine electrolyte to the cathode, and causes electrolysis of the brine and the formation of gaseous chlorine, which is removed from the cell, purified and stored. Elemental sodium, another product of the electrolysis, forms an amalgam with the mercury cathode, and is removed from the cell and processed to form a caustic solution. Regenerated mercury from the amalgam is recycled for use as the cell cathode.

Because of the lack of uniformity of the surface of the cell bottom and the pressure-of impurities which may adhere to the cell bottom, the mercury cathode is not of uniform height throughout the length of the cell bottom. It is therefore extremely difficult to position the anodes at a distance from the cathode which provides optimum electrolysis of the brine during dell operation.

Numerous techniques have been developed in an effort to determine the optimum distance between the anode and liquid cathode in an electrolytic cell. For example, U.S. Pat. No. 3,361,654, which issued to Charles Deprez et al. on Jan. 2, 1968 describes a method for adjusting graphite anodes by comparing the change of current with time as the anode is lowered towards the cathode. When there is a sharp increase in current as the anode is lowered, a point of incipient short circuit is reached, lowering of the anode is stopped and the anode is moved in the opposite direction from the cathode for a short distance. Although this technique may be satisfactory for positioning graphite anodes, it cannot be utilized when metallic anodes are positioned in the cell because contact between the metal anodes and the mercury cathode, unlike graphite anodes, markedly changes the characteristic of the metal anodes. In addition, the incipient short circuit disturbs the mercury and the thickness on the cell bottom may be changed as a result of this technique. As

It is a primary object of this invention to provide an improved method of adjusting anodes in an electrolytic cell.

Another object of this invention is to provide an improved method of automatically adjusting metal anodes to an optimum height in an electrolytic cell utilizing a liquid cathode.

These and other objects of the invention will be apparent from the following detailed description thereof.

It has now been discovered that the foregoing objects are accomplished in an electrolytic cell having adjustable anodes, an aqueous electrolyte and a liquid cathode wherein a voltage is applied across said anodes and said cathode to develop an electric current from said anodes through said aqueous electrolyte to said cathode, by utilizing the improved method for positioning a minor fraction of said anodes at an optimum distance from said cathode which comprises a. positioning a minor fraction of the anodes in the cell above the cathode at a distance apart so that when this fraction of the anodes is moved slightly in either direction, there is relatively small change in the current passing through the minor fraction of anodes,

b. moving the minor fraction of anodes in the direction of the cathode at a substantially constant rate,

0. measuring the current passing through the minor fraction of anodes and, based upon the current measurement, calculating the change in current density per unit of distance as the minor fraction of anodes moves towards the cathode, and

d. when the change in current density per unit of distance reaches the desired limits, for example, in the LII range between about 2 and about 10 kiloamperes per square meter per millimeter, movement of the minor fraction of anodes is discontinued.

The thus adjusted minor fraction of anodes is considered to be at a known anode-cathode gap which is near or at the optimum position for the most economic operation of the cell. Another minor fraction of anodes in the same cell is then adjusted to the optimum position in the same manner, and this procedure is continued until all of the anodes within the same cell have been placed at or near the optimum position. A continuous scan of the current and voltage measurements of the anodes can then be obtained, if desired, and further movement of the anodes within the cell can thenbe made in order to allow for any changes in the mercury thickness or characteristics of each set of anodes in each cell as operation continues.

FIG. 1 is a block diagram showing generally the layout of the apparatus useful in carrying out the process of this invention.

F IG. 2 is a block diagram showing the signal selection and signal conditioning system of the apparatus useful in carrying out the process of this invention.

FIG. 3 is a curve showing a typical relationship between the rate of change of current density per square meter per millimeter with change in-gap distance in millimeters for an electrolytic mercury cell.

FIG. 1 illustrates in block diagram form one embodiment of the apparatus useful in carrying out the process of this invention where current'and voltage signals 1 from each anode set (not shown) for each electrolytic cell 2 may be selected by cell selector unit 3. Anode set selector unit 4 can select the current and voltage signals from any anode set for electrolytic'cell 2 selected by cell selector unit 3. Manual control unit 5 and automatic control unit 6 can independently select current and voltage signals from 3 or 4,and perform the required operations. Motor control unit 7 for increasing or decreasing the anode-cathode spacing in any anode set in electrolytic cell 2 can be controlled by manual control unit or automatic control unit 6.

FIG. 2 represents the signal selection and conditioning system for a corresponding pair of anode sets in two adjacent electrolyteic cells in series. Conductor 8 supplies current to anode set 9 in cell 10 while conductor 11 supplies current to anode set 12 for cell 13. Terminals l4 and 15 along conductor 8 generate a signal representing the current flow to anode set 9. Similarly terminals 17 and 18 generate a current signal along conductor 11 representing the current flow to anode set 12. Thermistor circuits l6 and 19 provide temperature compensated current signals between terminals 14 and 15, and 17 and 18, respectively. Voltage signals across 9 and across 12 are generated between terminals 14 and 17, and 17 and 20, respectively. Amplifier 27 receives current signals from anode set 9 across relay circuits 21 and 22 and from anode set 12 across relay circuits 24 and 25. Amplifier 28 receives voltage signals for anode set 9 across relay circuit 21 and 23 and for anode set 12 across relay circuit 24 and 26. Chopper 29 receives the amplified current signal from 27 while chopper 30 receives the amplified voltage signal from 28 and these signals are converted from direct current to alternating current. Transformers 31 and 32 receive the converted current and voltage signals respectively and isolate the signals providing one at earth potential and another at cell potential. Detectors 33 and 34 convert the isolated current and voltage signals respectively from alternating to direct current. Gated integrators 35 and 36 receive the DC current and voltage signals from 33 and 34 and reject electrical noise, particularly that generated by the rectifier that supplies current to the electrolytic cells. Noise conditioned current and voltage signals are transmitted to hold units 37 and 38, respectively, to await selection by selector 39. Upon selection, the signals are converted from analog to binary form in converter 40 and transmitted to automatic control unit 6 for processing.

FIG. 3 shows the relationship of the rate of change in current density in terms of kiloamperes per square meter per millimeter of gap with the change in gap or distance between the anode and cathode in millimeters for a typical anode set. A sharp increase in the current density at about 1.0 millimeter gap shows that the optimum position is being approached for this anode set without obtaining an undesirable incipient short circult.

More in detail, the method of the present invention may be used on a variety of electrolytic cell types used for different electrolysis systems. It is particularly useful in the electrolysis of alkali metal chlorides to produce chlorine and alkali metal hydroxides. More particularly, it is highly suitable for horizontal electrolytic cells having a liquid metal cathode such as mercury, as disclosed, for example in US. Pat. Nos. 3,390,070 and 3,574,073, which are hereby incorporated by reference in their entirety.

As indicated in US. Pat. No. 3,574,073, issued Apr. 6, 1971 to Richard W. Ralston, Jr., horizontal mercury cells usually consist of a covered elongated trough sloping slightly towards one end. The cathode is a flowing layer of mercury which is introduced at the higher end of the cell and flows along the bottom of the cell toward the lower end. The anodes are generally composed of rectangular blocks of graphite or titanium rods coated with a metal oxide. The anodes are suspended from conductive lead-ins, for example, graphite or protected copper tubes or rods. The bottoms of the anodes are spaced a short distance above the flowing mercury cathode during cell operation. The electrolyte, which is usually salt brine, flows above the mercury cathode and also contacts the anode. For each anode in each set, one anode lead is secured to a conductor, and the other lead is secured to a second conductor. Each conductor is adjustably secured at each end to a supporting post. Each supporting post is provided with a drive means such as a sprocket attached to the upper portion thereof, which is driven through a belt or chain, indirectly or directly, by a motor such as an electric motor, hydraulic motor or other motor capable of responding to electric signals.

Although the invention is particularly useful in the operation of horizontal mercury cells used in the electrolysis of brine, it is generally useful for any liquid cathode type electrolytic cell where adjustment of the anode-cathode space is necessary.

The number of electrolytic cells controlled by the method of this invention is not critical. Although a single electrolytic cell can be controlled, commercial operations containing more than cells can be successfully controlled. In carrying out the method of this invention, each electrolytic cell contains at least about four, and up to about 200 anodes per cell, and preferably from about l0 to about 100 anodes per cell.

It is preferred, particularly on a commercial scale to utilize an anode set as the minor fraction of anodes when adjusting the space between the cathode and anodes of electrolytic cells. An anode set may contain a single anode, but it is preferred to include from 2 to about 20 anodes and preferably between from about 3 and about 12 anodes per anode set. Each anode has at least one conductor, but preferably has at least two conductors. Voltage and current measurements are obtained for each individual anode set in each cell.

Current to any part of the cell is calculated by the formula where I current in kiloamperes E voltage difference between the measured voltage and the decomposition voltage which is 3.1 volts for salf brine in a mercury cell G gap in millimeters between the top of the mercury cathode and the bottom of the anode K, Resistivity of the brine K All other resistances including conductors, po-

larization, and the like.

In a mercury cell used for salt brine electrolysis, K is about 0.000019 Q/M /MM, and K generally ranges from about 0.00004 to about 0.00006 Q/M? When a minor fraction of the anode is moved toward the cathode the voltage (E) stays relatively constant because, although the current can increase greatly in the minor fraction of anodes, the total current in the rest of the cell is much greater than in the minor fraction of anodes and the relative decrease is small.

The rate at which the current to a minor fraction of anodes changes as the anode is moved toward the cathode is shown in FIG. 3. Using the above equation the rate of change can be calculated to be:

The curve in FIG. 3 shows the rate of change of current with varying gap based on experimental data for metal anodes in a mercury cell. As the gap is decreased to less than about 1 mm the rate of change increases sharply.

It can be seen that the method is very sensitive in determining when this gap is reached. With this method it is possible to determine when the anode is close to the mercury without lowering the anode until incipient short circuiting occurs.

The minor fraction of anodes which are lowered in accordance with the process of this invention may range from about 2.5 to about 25 percent of the total anodes in the cell, and preferably from about 5 to about percent of the total anode in the cell. When a minor fraction of the anodes is lowered in this manner, there is initially very little change in the current density through the anode set as the anode set is moved progressively towards the cathode. A computer, such as a digital computer, is utilized, on signals received from the minor fraction of anodes being lowered to measure voltage, current and calculate current density per square meter of anode surface per'millimeter as the distance between the cathode and the anode is decreased. The initial current density increases relatively slowly incipient short circuit, which will adversely affect cell operation and severly damage metal anodes, when metal anodes are employed.

The rate of lowering of the anode set during this ad- 5 justment period is generally at the rate of from about lower the anode set at a rate exceeding the aboveuntil the anode approaches the mercury cathode.

When this occurs, there is a marked increase in the current density. For example, in a commercial electrolytic mercury cell containing 10 sets of 5 anodes, each anode having a surface area of about 4 ft. by 9, in., when one anode set is lowered from about the 3 millimeters gap position for a distance of about 0.1 millimeter, the current density may increase about 3.5 percent. This relatively small increase continues until the anode set is only about 1 millimeter from the cathode. At this point a movement of an additional 0.1 millimeter will cause the current density to increase by about 10 percent, or more. For example, when the anode is about 3 millimeters from the cathode the rate of increase of current density is about one kiloampere per square meter per millimeter. When the anode is further moved towards the cathode to a gap of about 1 millimeter, the rate of current density change rapidly increases to about 3.5 kiloamperes per square meter per millimeter.

Additional movement of the anode set towards the anode to a gap of about 0.5 millimeters causes the rate of current density change to increase to about 5 kiloamperes per square meter per millimeter. Thus it can be seen that adjustment of the anode set to a point where the rate of current density increase ranges from about 2 to 10 and preferably from about 3 to about 5 kiloamperes per square meter per millimeter will position the anode at about 0.5 to about 1.0 mm from the cathode. Further adjustment from this position can be made if desired. When the rate of current density change is less than about 0.5 kiloamperes per square meter per millimeter, operation of the cell under those conditions is less economical and when the rate of current density change exceeds about 10 kiloamperes per square meter per millimeter the gap is so small that there is a risk of contacting the cathode and causing an mentioned rates. of descent. Furthermore, appropriate signals are programmed into the computer to limit the total distance that the anode set is lowered to a point where the current density will not exceed previously determined limits, for example, not in excess of about 10 kiloamperes per millimeter per second for a cell of the type described above.

After the initial minor fraction of anodes in the cell has been adjusted in this manner, a second anode set is then subjected to the same procedure to determine the optimum gap. This procedure is then repeated until all the anode sets in the cell have been adjusted to the optimum position.

Although FIG. 3 presents a typical curve for a commercial type cell and anodes of this type cell will follow the general shape of the curve, it will be recognized by those skilled in the art that the variations in size and shape of the cell and electrodes may change the posi- In accomplishing the adjustment of the anode-' cathode spacing by the method of the present invention, two electrical signals are generated and measured for each anode set. One corresponds to the current flow in the conductor for the anode set and may be obtained by measuring thevoltage drop between a plurality of terminals, preferably two, spaced apart a suitable distance along the conductor. The spacing suitably vaties between 3 and inches,for example about 30 inches, but should be the same distance for all conductors. It is desirable, but not essential, that the terminals be located laterally in the middle of the conductor, in a straight segment of conductor of uniform dimensions. Current measurements may also be obtained using other well known methods such as by the Hall effect or other magnetic detection devices.

The current signal may be compensated for temperature changes in the conductor by a thermal resistor being embedded or otherwise attached to the section of conductor being used as the source of the current signal.

The voltage signal is generated and measured between corresponding terminals on the conductors for corresponding anode sets on two adjacent cells when a multiplicity of cells are controlled.

The novel process of this invention can be used in combination with other techniques for adjusting the anode-cathode spacing in an electrolytic cell having at least one anode set. For example, in response to an electric signal from the manual or automatic control unit, the voltage drop is measured between a plurality of terminals along a section of each conductor supplying current to the anode set. This electrical signal represents the current flow in the conductor for the anode set. A voltage signal is measured across the anode set, generally between corresponding terminals on the conductors for two adjacent anode sets in cells having a plurality of anode sets. These electrical signals are processed and used to calculate the voltage coefficient Vc according to the formula:

where E is the voltage defined above, V is the measured voltage for the electrolytic unit such. as an anode set; D is the decomposition voltage for the electrolysis being conducted, and KA/M is the current density in kiloamperes per square meter of cathode surface below the anode set. In the electrolysis of sodium chloride in a mercury cell for producing chlorine, the value for D is about 3.1.

The calculated voltage coefficient Vc during operation is compared with an original standard voltage coefticient S which was calculated at start-up in accordance with the above formula for voltage coefficient, Vc. Standard voltage efficient S may vary with a number of factors such as the material of construction of the anode (graphite or metal), the form and condition of the anodes (blocks of graphite which are slotted or K, standard volta e coefficient, V/k Condition 0.55 no device for regulating anode position 0.3 use of device for lowering anodes 0.2 intensive perforation of the anodes 0.14 increased perforation of anodes and increased electrolysis temperature 0.09 use of titanium anodes with ruthenium dioxide coating 0.022 anodes specially placed in the amalgam.

This standard voltage coefficient is suitably raised or lowered as the condition of the anode set changes. It can be stored in the memory core of a computer and compared with the calculated Vc when desired.

Upon comparing calculated coefficient V with predetermined standard coefficient S for a selected anode set, if the value falls outside of k, where k is the permissible range of deviation from S, an adjustment of the anode-cathode spacing is made. The value of k may vary from about 0.1 to about percent, preferably from about 2 to about 8 percent of S.

Where the difference between Vc and S falls outside the range k the minor fraction of anodes, or anode sets, is readjusted to determine a new standard voltage coefficient S for the anode set.

The spacing of the anode set is first increased a predetermined amount to insure that the spacing is above about 3 mm. The minor fraction of anodes is then lowered at a substantially constant rate within the above defined range for rate of lowering and the rate at which the current increases is determined as the anode set is being lowered. When the rate reaches a value indicating a gap of about 0.5 to about 1 mm., for example, (about 3 KA/MIMM) movement of the anode set is stopped.

The current readings are then analyzed for conditions indicating that the anode set is too close to the cathode.

One analysis determines whether the current continues to increase for greater than a predetermined time period, for example about 4 seconds, and if so the anode-cathode spacing is immediately increased.

Another analysis determines whether the total increase in current after the anode set has stopped moving, exceeds a predetermined limit, for example, an increase of 6-8 percent, and if so the anode-cathode spacing is immediately increased.

if the first two current analyses do not indicate close approach of the anode to the cathode, a series of N current measurements are taken. An analysis is made to determine the magnitude of current fluctuations over a predetermined time period. The average magnitude of the current fluctuations is determined by any convenient method prior to comparing with a predetermined average difference limit. The term average difference when used in the description and claims to define the magnitude of the current fluctuations is intended to include any known method of averaging differences. For example, in a preferred embodiment a calculation is made for 2 MIN, where A is the difference in current between each successive reading and N is the total number of current readings taken. If this number is greater than a predetermined average difference limit, for example, greater than about 1.0, the anode-cathode spacing is immediately increased. As an alternate, the avera e difference may be obtained by the calculation V yA /N or any other similar statistical technique.

If these analyses do not show extremely close approach or incipient short circuit the indicated gap is generally in the range of about /a to 1 mm. This is considered to be near the optimum gap for operation of the cell. However, if this position is too close for stable long term operation, the anode may be raised, for example, about 1 mm for a resulting gap in the range of about 1 /2 to 2 mm.

More readings of current and'voltage are then taken and the value for Vc at this position is calculated. The standard voltage coefficient, S is then reset to the later Vc value for this resulting gap, providing the new S is not outside the range of M which is equal to 20 percent less up to percent more than original S. If the new S exceeds this range the computer is programmed to provide asignal that consideration should be given for replacement of the anode set. If the new S is below this range the computer is programmed to raise the anode set until the new S is within the above mentioned-described range M for S.

Cells and anode sets within a cell can be selected for adjustment randomly or serially and the selection can be made manuallyor by automatic control. Upon selection of a given cell for adjustment all current and voltage signals for the anode sets in the cell are taken and processed simultaneously to give readings which are truly comparable. Likewise, cells and anode sets may be omitted from the adjustment method.

In another embodiment of the method of the present invention, the anode-cathode spacing for an anode set is controlled by periodically measuring the current to an anode set, comparing the current reading with a predetermined standard and increasing the anode-cathode spacing when the standard is exceeded by a predefined limit.

In a further embodiment of the method of the present invention, the frequency of changing the anode cathode spacing for an anode set during a predefined time period is determined and the anode set is removed from automatic control when the frequency of change exceeds a predetermined number.

When a cell is to be selected, either manually or automatically, for determining whether any or all anode sets in the cell require adjustment, a scanner unit selects the relay for the desired cell and all of the current and voltage signals for that cell are received by the scanner and passed through an amplifier where the signals are amplified.

For automatic adjustment from the amplifier, the signals are conditioned for use by the computer. This may be done, for example, by passing the signals through a chopper which converts direct current (DC) signals to alternating current (AC) signals, then to a transformer which isolates the signals, providing one at earth potential and another at cell potential and then to a detector which converts AC signals back to DCsignals. The DC signals are transmitted to a gated integrator which rejects electrical noise, particularly that generated by the rectifier. The frequency of noise rejected is dependent on the cycle of current used in the cell plant. For example, if 60 cycle current is being used, the integrator rejects noise at 60 cycles and all harmonics. Noise conditioned signals are transmitted to a hold unit (capacitator) to be retained until required for use invarious calculations. Where these calculations are to be made by a digital computer, conversion of the signals from analog to digital form is required.

When a cell or anode set is selected for adjustment, a series of current and voltage signals are simultaneously generated for a controlled time period, for example, from about 0.25 to about seconds, at a rate of from about to about 60 per second. Any suitable time period and rate may be, used.

In a further embodiment used with the method of the present invention, all anode sets for all cells in operation are serially scanned periodically and the current and voltage readings for each anode set compared with its predetermined limits. Where the current reading exceeds predetermined limits, the anode-cathode spacing about 580 anodesets, and any suitable interval between scans may be selected, for example, one minute. If during a scan, the anode-cathode spacing for an anode set is increased, the scan may be repeated for all anode sets for all operative cells.

An further embodiment used with the method of the present invention comprises counting the frequency of change in the anode-cathode spacing for a particular anode set during a predetermined time period and where this frequency exceeds a predetermined number, to remove this anode set from automatic control. This may be indicated for example, by the sounding of an alarm, activating a light on a control panel or causing a message to be printed out on a reader-printer unit associated with a computer.

The following example is presented to define the invention more fully. All parts and percentages are by weight unless otherwise specified. I

EXAMPLE I A horizontal mercury cathode cell for electrolyzing aqueous sodium chloride to produce chlorine contain-' ing 10 anode sets of 5 metal anodes per set was selected by the cell scanner unit. Current and voltage signals for all 10 anode sets were received simultaneously for about 5 seconds until about readings of current and of voltage were received for each anode set. The average voltage, current, and the difference between each current reading and the previous current reading was determined by a digital computer for the series of readings. The voltage coefficient was calculated for each anode set according to the formula:

Anode set 2, with a cathode surface area of 2.4 square meters, was found to have a Vc of 0.140 based on an average voltage of 4.3 and an average current of 18.86 kiloamperes. When Vc was compared with the standard coefficient S of 0.1 15 for anode set 2, it was found to have a value above the deviation range k, where k was i 5 percent. Original S was 0.1 10.

When the coefficient comparison determined the value of Vc was above S by a value greater than k, a signal from the computer activated a relay which energized a motor to increase the anode-cathode spacing by 3 mm.

The computer then activiated a relay which energized the motor to decrease the anode-cathode gap at a rate of about 0.3 mm per second. It then took read? ings at a rate of 50 per second and determined the rate of increase in current density by comparing the most recent reading with the tenth prior reading to reduce the effect of signal noise. When the rate of increase reached 3 KA per M per mm (an estimated 1 mm anode-cathode gap) the anode set'was stopped at this position. A number of measurements were made and analyzed as follows:

1. Analyzed current measurements to determine whether current continued to increase over a period of about 4 seconds. The increase incurrent stopped before the expirationof the 4 second period. No change in the anode set position was made because of this analysis.

2. The total increase in current during that period was less than about percent. No change in the anode position was made because of this analysis.

3. More than 100 current measurements were analyzed for current fluctuation determined by the formula: ZA N. The fluctuation was found to fall within the predetermined limit of 0.5 percent, which indicated that the anode-cathode gap was stable and that the anode could remain at this position.

4. Voltage coefficient, Vc, was calculated to be 0.1 which was within the allowable range M for S of from 0.080 to 0.200. The standard voltage coefficient S for this anode set was then reset to the new value of 0.110.

5. Readings were then taken for all anode sets on the cell and the Vc calculated for each was found to have a value within 5 percent of the stored values of S. No further adjustments were made and the next cell to be adjusted was selected.

What is claimed is:

1. In an electrolytic cell containing adjustable anodes, a liquid cathode and an aqueous electrolyte wherein a voltage is applied across saidanodes and said cathode to develop an electric current from said anodes through said aqueous electrolyte to said cathode, the improved method for positioning a minor fraction of said anodes at an optimum distance from said cathode which comprises a. positioning a minor fraction of said anodes above said cathode at a distance apart so that when said minor fraction of anodes is moved in either direction an incremental distance, there is a relatively small change in current passing through said minor fraction of anodes,

b. moving said minor fraction of anodes in the direction of said cathode at a substantially constant rate,

c. measuring said current through said minor fraction of anodes and calculating the current density per unit of distance as said minor fraction of anodes moves toward said cathode, and

d. discontinuing movement of said minor fraction of anodes towards said cathode when the change in said current density per unit of distance reaches a predetermined limit.

2. The method of claim 1 wherein said predetermined limit is within the range from about 2 and about 10 kiloamperes per square meter per millimeter.

3. The method of claim 2 wherein said predetermined limit is in the range from about 3 to about 5 kiloamperes per square meter per millimeter.

4. The method of claim 2 wherein said adjustable anodes are metal anodes.

5. The method of claim 2 wherein said adjustable anodes are graphite anodes.

6. The method of claim 4 wherein said minor fraction of anodes is in the range from about 2.5 to about 25 percent of the total anodes in the cell.

7. The method of claim 6 wherein said minor fraction of anodes is the range between about 5 and about percent of the total anodes in the cell.

8. The method of claim 7 wherein all of the anodes in the cell are positioned above the optimum distance before adjusting said minor fraction.

9. The method of claim 7 wherein said minor fraction of anodes is moved in the direction of said cathode at the rate of between about 0.1 and about 1.0 millimeter per second.

10. The method of claim 9 wherein said rate of change in current density for said minor fraction of anodes is calculated by a digital computer which in response to electric signals, senses current anddistance moved by said minor fraction of anodes, calculates the change in current density per unit of distance, and stops movement of said minor fraction of anodes when the current density exceeds said predetermined limit.

11. The method of claim 10 wherein said minor fraction of anodes is moved by anode supports having sprockets attached to the upper portion thereof, said sprockets being moved by a chain cooperating with said sprockets and a motor driven gear.

12. The method of claim 11 wherein said motor driven gear is energized by a motor controlled by said digital computer.

13. The process of claim 1 wherein said minor fractions of anodes is raised a predetermined height to improve long term operations of the cell.

14. The process of claim 13 wherein said predetermined height is in the range from about 0.5 to about 1.5

15. The process of claim 1 wherein said improved method comprises a. calculating at start-up an original standard voltage coefficient S, for a minor fraction of said anodes in accordance with the formula:

where:

l. V is the voltage across said minor fraction of anodes, 2. D is the decomposition voltage of said electrolyte, 43. KA is the current to said minor fraction of anodes, and, 4. M is the area in square meters of the cathode surface below said minor fraction of anodes, b. continuing operation of the cell for a predetermined period, c. measuring the voltage across said minor fraction of anodes,

d. measuring the current to said minor fraction of anodes,

e. calculating the voltage coefficient Vc according to said formula,

f. comparing said Vc with said standard voltage coefficient S for said minor fraction of anodes,

g. adjusting the space between said minor fraction of anodes and said cathode where the difference between Vc and 5 falls outside of k, where l. k is the permissible range difference between Vc and S for said minor fraction of anodes, 2. wherein said adjusting comprises a. increasing the space between said minor fraction of anodes and said cathode by a distance of above about 3 millimeters,

h. moving said minor fraction of anodes in the direction of said cathode at a substantially constant rate,

i. measuring said current through said minor fraction of anodes and calculating the current density per unit of distance as said minor fraction of anodes moves toward said cathode, and

j. discontinuing movement of said minor fraction of anodes towards said cathode when the change in said current densityper unit of distance reaches a predetermined limit.

16. The process of claim 15, after said discontinuing movement, wherein said improvement also comprises measuring the current to said minor fraction of anodes for a predetermined period, and raising said minor fraction of anodes when the current during the predetermined period increases beyond a predetermined limit.

17. The process of claim 16, after measuring current for said predetermined period, wherein said improvement also comprises comparing the maximum current measurement during said predetermined period with the current initially attained when said minor fraction of anodes was stopped, and raising said minor fraction of anodes when the difference between the initial current and the maximum current attained during said predetermined period exceeds a predetermined limit.

18. The process of claim 16, after measuring current for said predetermined period, wherein said improvement also comprises calculating the average difference of said current measurements obtained during said predetermined period and raising said minor fraction of anodes when said average difference exceeds a predetermined limit.

19. The process of claim 15, after said discontinuing movement, where in said improvement also comprises,

a. measuring voltage and current to said minor fraction of anodes and calculating a new voltage coefficient Vc for the resulting new position,

b. comparing said new voltage corefficient Vc with original standard voltage coefficient S,

c. changing standard voltage coefficient S to said new voltage coefficient Vc, when the difference between said new voltage coefficient Vc and said original standard coefficient S exceeds a predetermined limit. 

1. IN AN ELECTROLYTIC CELL CONTAINING ADJUSTABLE ANODES, A LIQUID CATHODE AND AN AQUEOUS ELECTROLYTE WHEREIN A VOLTAGE IS APPLIED ACROSS SAID ANODES AND SAID CATHODE TO DEVELOP AN ELECTRIC CURRENT FROM SAID ANODES THROUGH SAID AQUEOUS ELECTROLYTE TO SAID CATHODE, THE IMPROVED METHOD FOR POSITIONING A MINOR FRACTION OF SAID ANODES AT AN OPTIMUM DISTANCE FROM SAID CATHODE WHICH COMPRISES A. POSITIONING A MINOR FRACTION OF SAID ANODES ABOVE SAID CATHODE AT A DISTANCE APART SO THAT WHEN SAID MINOR FRACTION OF ANODES IS MOVED IN EITHER DIRECTION AN INCREMENTAL DISTANCE, THERE IS A RELATIVELY SMALL CHANGE IN CURRENT PASSING THROUGH SAID MINOR FRACTION OF ANODES, B. MOVING SAID MINOR FRACTION OF ANODES IN THE DIRECTION OF SAID CATHODE AT A SUBSTANTIALLY CONSTANT RATE, C. MEASURING SAID CURRENT THROUGH SAID MINOR FRACTION OF ANODES AND CALCULATING THE CURRENT DENSITY PER UNIT OF DISTANCE AS SAID MINOR FRACTION OF ANODES MOVES TOWARD SAID CATHODE, AND D. DISCONTINUING MOVEMENT OF SAID MINOR FRACTION OF ANODES TOWARDS SAID CATHODE WHEN THE CHANGE IN SAID CURRENT DENSITY, PER UNIT OF DISTANCE REACHES A PREDETERMINED LIMIT.
 2. The method of claim 1 wherein said predetermined limit is within the range from about 2 and about 10 kiloamperes per square meter per millimeter.
 2. D is the decomposition voltage of said electrolyte,
 2. wherein said adjusting comprises a. increasing the space between said minor fraction of anodes and said cathode by a distance of above about 3 millimeters, h. moving said minor fraction of anodes in the direction of said cathode at a substantially constant rate, i. measuring said current through said minor fraction of anodes and calculating the current density per unit of distance as said minor fraction of anodes moves toward said cathode, and j. discontinuing movement of said minor fraction of anodes towards said cathode when the change in said current density per unit of distance reaches a predetermined limit.
 3. The method of claim 2 wherein said predetermined limit is in the range from about 3 to about 5 kiloamperes per square meter per millimeter.
 4. The method of claim 2 wherein said adjustable anodes are metal anodes.
 4. M2 is the area in square meters of the cathode surface below said minor fraction of anodes, b. continuing operation of the cell for a predetermined period, c. measuring the voltage across said minor fraction of anodes, d. measuring the current to said minor fraction of anodes, e. calculating the voltage coefficient Vc according to said formula, f. comparing said Vc with said standard voltage coefficient S for said minor fraction of anodes, g. adjusting the space between said minor fraction of anodes and said cathode where the difference between Vc and S falls outside of k, where
 5. The method of claim 2 wherein said adjustable anodes are graphite anodes.
 6. The method of claim 4 wherein said minor fraction of anodes is in the range from about 2.5 to about 25 percent of the total anodes in the cell.
 7. The method of claim 6 wherein said minor fraction of anodes is the range between about 5 and about 15 percent of the total anodes in the cell.
 8. The method of claim 7 wherein all of the anodes in the cell are positioned above the optimum distance before adjusting said minor fraction.
 9. The method of claim 7 wherein said minor fraction of anodes is moved in the direction of said cathode at the rate of between about 0.1 and about 1.0 millimeter per second.
 10. The method of claim 9 wherein said rate of change in current density for said minor fraction of anodes is calculated by a digital computer which in response to electric signals, senses current and distance moved by said minor fraction of anodes, calculates the change in current density per unit of distance, and stops movement of said minor fraction of anodes when the current density exceeds said predetermined limit.
 11. The method of claim 10 wherein said minor fraction of anodes is moved by anode supports having sprockets attached to the upper portion thereof, said sprockets being moved by a chain cooperating with said sprockets and a motor driven gear.
 12. The method of claim 11 wherein said motor driven gear is energized by a motor controlled by said digital computer.
 13. The process of claim 1 wherein said minor fractions of anodes is raised a predetermined height to improve long term operations of the cell.
 14. The process of claim 13 wherein said predetermined height is in the range from about 0.5 to about 1.5 mm.
 15. The Process of claim 1 wherein said improved method comprises a. calculating at start-up an original standard voltage coefficient S, for a minor fraction of said anodes in accordance with the formula:
 16. The process of claim 15, after said discontinuing movement, wherein said improvement also comprises measuring the current to said minor fraction of anodes for a predetermined period, and raising said minor fraction of anodes when the current during the predetermined period increases beyond a predetermined limit.
 17. The process of claim 16, after measuring current for said predetermined period, wherein said improvement also comprises comparing the maximum current measurement during said predetermined period with the current initially attained when said minor fraction of anodes was stopped, and raising said minor fraction of anodes when the difference between the initial current and the maximum current attained during said predetermined period exceeds a predetermined limit.
 18. The process of claim 16, after measuring current for said predetermined period, wherein said improvement also comprises calculating the average difference of said current measurements obtained during said predetermined period and raising said minor fraction of anodes when said average difference exceeds a predetermined limit.
 19. The process of claim 15, after said discontinuing movement, where in said improvement also comprises, a. measuring voltage and current to said minor fraction of anodes and calculating a new voltage coefficient Vc for the resulting new position, b. comparing said new voltage corefficient Vc with original standard voltage coefficient S, c. changing standard voltage coefficient S to said new voltage coefficient Vc, when the difference between said new voltage coefficient Vc and said original standard coefficient S exceeds a predetermined limit.
 43. KA is the current to said minor fraction of anodes, and, 